Synthesis and properties of the water-soluble self-acid-doped polypyrrole: poly[4-(3-pyrrolyl)butanesulfonic acid]

The water-soluble polypyrrole, poly[4-(3-pyrrolyl)butanesulfonic acid] (P3PyBSH), in which the β-position of pyrrole ring is substituted with n-butanesulfonic acid group, is synthesized by oxidation polymerization of 3PyBSNa followed with an ion-exchange of Na⁺ with H⁺. It can be cast into free-standing film from its aqueous solution and has a conductivity of 4.0×10^-4 S/cm, higher than that of its sodium salt (5.5×10^-6 S/cm) by about 2 orders. The increase in conductivity is attributed to the self-doping by the protons originally on the side chains as supported by the absence of the anion FeCl₄^- (which generates during the polymerization) and the presence of polaron and bipolaron characteristics in its UV-Vis and IR spectra.